A polycarbonate (PC) is excellent in heat resistance, impact resistance and transparency, so that it has widely been used in many fields in recent years. With regard to the producing method of the polycarbonate, a lot of investigation has conventionally been performed. Among these, the producing methods of a polycarbonate derived from an aromatic dihydroxy compound, for example, 2,2-bis(4-hydroxyphenyl)propane (hereinafter referred to as “bisphenol A”), has been industrialized by either the interfacial polymerization method or the melt polymerization method.
The polycarbonate produced by the interfacial polymerization method is manufactured from bisphenol A, an aromatic monohydroxy compound such as p-tert-butylphenol, etc., and phosgene, which can be produced under relatively low temperature conditions, so that the obtained polycarbonate is generally a linear polymer and shows Newtonian when it is melted. That is, with regard to shear fluidity, shear velocity dependency on the melt viscosity is small, and with regard to elongation fluidity, an extremely low viscosity is shown, so that when a large-sized extrusion molding or blow molding is carried out, dripping of the resin by self-weight is likely occurred, whereby molding of a large-sized product is difficult.
Melting characteristics of a polycarbonate resin can be shown by Q=K·PN (wherein Q represents a flow amount (ml/sec) of a melt resin, K represents an intercept of a regression equation and is an independent variable (derived from a molecular weight or a structure of the polycarbonate resin), P represents a pressure (load: 10 to 160 kgf) (kg/cm2) measured at 280° C. using a Koka-type flow tester, and N is a structural viscosity index). In the formula, when N=1, it shows Newtonian flow behavior, and as the N value becomes large, pressure dependency of the fluidity becomes large and non-Newtonian flow behavior is shown. A polycarbonate resin to be used for the uses such as a large capacity hollow molding article, a large-sized extrusion molding product, a blow molding product, etc., is evaluated its melt-flow characteristics by the N value, and that showing non-Newtonian flow behavior in which pressure dependency is large is generally preferred since dripping or draw down at the time of extrusion or molding can be prevented. Accordingly, it has been desired to optionally produce a polycarbonate resin having suitable melt-flow characteristics in which the N value is in the proper range.
Thus, in the interfacial polymerization method in general, non-Newtonian at the time of melting is controlled by adding a polycarbonate resin component having an extremely high molecular weight, or forming a branched structure by optionally incorporating the branching agent into the molecules. That is, improvement in blow moldability, drip-preventing performance and flame retardance, etc., has been carried out by optionally increasing a melt viscosity or an elongation viscosity at a low shear velocity region.
The reason why such a matter is possible is that, in the interfacial polymerization method, there is a correlation with a certain extent between an amount of the branching agent to be used and a branching degree, and the branching degree can be optionally controlled by the use of an optional amount of the branching agent. However, in the interfacial polymerization method, poisonous phosgene must be used in the producing method. In addition, there remains the problems that the apparatus is corroded by the by-producing hydrogen chloride or sodium chloride and a chlorine-containing compound such as methylene chloride, etc., used as a solvent with a large amount, and it is difficult to remove impurities such as sodium chloride, etc., or remaining methylene chloride, that may affect the physical properties of the polymer.
On the other hand, the melt polymerization method which has conventionally been known as another producing method of the polycarbonate resin is a method for producing a polycarbonate from an aromatic dihydroxy compound and a diarylcarbonate, and, for example, bisphenol A (BPA) and diphenylcarbonate (DPC) are subjected to transesterification reaction in a molten state, and polymerized while removing a by-produced aromatic monohydroxy compound.
The melt polymerization method has a merit that no solvent is used, etc., different from that of the interfacial polymerization method, but in the producing process, it is necessary to carry out the reaction at high temperature and under highly vacuum state for a long period of time to distill off the aromatic monohydroxy compound or the carbonic acid diester in the high viscosity polycarbonate molten material. Accordingly, as a producing apparatus, it is necessary to manufacture a specific apparatus which can endure the reaction under high temperature and highly vacuum state for a long period of time, and a strong stirring device since the product has high viscosity.
Also, an unspecified amount of the branched structure is generated in the high molecular weight polycarbonate produced by the conventional transesterification method at the time of production, so that the branching degree cannot be expected at the time of melting. In addition, it shows larger non-Newtonian as compared with the case produced by the interfacial polymerization method. This branched structure is based on a branched structure or on a crosslinked structure due to an ester bond formed by subjecting the polycarbonate to similar reaction to Kolbe-Schmitt reaction by an action of an alkali, and it has been known that it is difficult to control the amount of the branched structure. That is, there is a possibility that it increases or decreases depending on the apparatus and operating conditions, whereby it is extremely difficult to control the flow behavior at the time of melting depending on the various kinds of moldings.
Also, hue of the high molecular weight polycarbonate produced by the conventional transesterification tend to be lowered, and a product which becomes tinged with yellow can only be industrially obtained. It has further been known that it has a defect that strength is inferior (brittle fracture is remarkable).
As the conventionally known method to solve the problem of lowering in hue, there is an attempt to shorten the time required for the reaction by controlling a charging molar ratio of the polymerization starting materials and heightening the polymerization rate. More specifically, a molar ratio of DPCs/BPAs at the time of charging the materials for the polymerization reaction is adjusted to obtain the stoichiometrically maximum polymerization rate. While the charging ratio is also affected by the characteristics of the polymerization reaction apparatus, it is possible to obtain a relatively high speed polymerization rate by setting it between, for example, 1.03 to 1.10.
According to this method, however, whereas effectiveness in a low molecular weight region can be confirmed, in a high molecular weight region, a polymerization reaction product becomes an extremely high viscose fluid and the polymerization rate becomes markedly slow, so that resin deterioration such as crosslinking, branching or lowering in hue, can be markedly observed by heat retention, etc., for a long period of time during the polymerization. Therefore, it was substantially extremely difficult to obtain a high molecular weight polycarbonate in which a branched-structure amount has been controlled to an optional amount by controlling a molar ratio of the charged polymerization starting materials. That is, when a polycarbonate resin is produced by using the melt polymerization method, it was extremely difficult to control melt viscosity or elongation viscosity in a low shear velocity region similarly in the interfacial polymerization method and to quantitatively improve blow moldability, drip-preventing performance and flame retardance, etc., only by the addition amount of the branching agent.
As a means to structurally improve the polycarbonate, it has been made an attempt that a naturally generating branched structure is to be reduced in the transesterification method polycarbonate. For example, it has been proposed a transesterification method polycarbonate which has no branched structure or decreases as much as possible in Patent Documents 1 and 2. Also, in Patent Document 3, a method for producing a polycarbonate in which the above-mentioned Kolbe-Schmitt type branched structural material is 300 ppm or less has been proposed.
Further, in Patent Documents 4 and 5, it has been proposed that formation of a branched structure by the side reaction control of which is extremely difficult is eliminated by using a specific catalyst, whereby color tone is improved and a specific branched structure is positively introduced by using a polyfunctional compound, and a transesterification method polycarbonate in which hollow moldability has been improved by increasing non-Newtonian of flow behavior is disclosed.
However, these methods use a specific compound as a catalyst, or methods in which a specific catalyst(s) is selected or used in combination, which cannot be said to be general, and further, an influence of the catalyst on a human body or the environment is concerned at the time of using the obtained polycarbonate.
In Patent Document 6, an attempt has also been made to improve molding fluidity using 5-(dimethyl-p-hydroxybenzyl)salicylic acid as a branching agent. However, use of the polyfunctional compound involves the problem that gel is likely formed by crosslinking. In Patent Documents 7 and 8, it has been proposed to control the branched structure derived from the above-mentioned Kolbe-Schmitt type heat deterioration within a certain range by using a specific apparatus, temperature range, and retention conditions. According to this controlling method, however, it is difficult to fundamentally suppress the spontaneous generation of the branched structure, and yet, it is a heterogeneous structure by a spontaneously generated heat deterioration reaction, so that it is necessary to use specific operating conditions in the specific apparatus for optionally controlling the generating amount of the branched structure. In Patent Document 9, an acid anhydride has been used as a branching agent, but effects on the physical property or color tone due to occurrence of the acid at the time of production or introduction of the ester bond, etc., cannot be ignored.
Accordingly, it has been desired to develop a method for producing a polycarbonate which is excellent in color tone or mechanical property, and can control flow behavior, non-Newtonian and molding fluidity similarly to that of the polycarbonate by the interfacial polymerization method, by a general transesterification method simply and easily, or a producing method of a polycarbonate obtained by a transesterification method, which can optionally control the branching degree and can obtain a polycarbonate having a desired branching degree simply and easily.
Also, as an improving means in the aspect of the process, in Patent Document 10, it has been proposed, in a continuous producing method in which some polymerization vessels are connected, to use a specific horizontal stirring polymerization vessel as the final polymerization vessel. Further, in Patent Documents 11 and 12, it has been proposed a method using a twin-screw vent extruder. However, these are intended to promote elimination of the phenol, and whereas a high molecular weight polycarbonate can be obtained thereby, the physical property thereof could not be satisfied in both of the mechanical properties and molding fluidity.
The present inventors have previously found a novel method which is to chain-elongate the sealed terminal of the aromatic polycarbonate by linking with an aliphatic diol compound, as a method for accomplishing a high speed polymerization rate and obtaining an aromatic polycarbonate having good quality (Patent Document 13). According to this method, the sealed terminal of the aromatic polycarbonate is chain-elongated by linking with the aliphatic diol compound, whereby a high polymerization degree aromatic polycarbonate resin having an Mw of about 30,000 to 100,000 can be produced within a short period of time. This method can produce the polycarbonate by a high speed polymerization reaction, so that branching and crosslinking reaction generated by heat retention, etc., for a long period of time can be suppressed, and resin deterioration such as lowering in hue, etc., can be avoided.
The present inventors have also found that a correlation with a certain extent can be established between the amount of the branching agent to be used and the branching degree of the obtained high molecular weight branched aromatic polycarbonate, when an aromatic polycarbonate prepolymer into which a branched structure has been introduced using a predetermined amount of a branching agent is reacted with an aliphatic diol compound to link to each other in the presence of a transesterification catalyst (Patent Document 14).
Incidentally, with regard to the producing method of a branched polycarbonate resin using a trifunctional or more of the polyol compound, it has been proposed a continuous producing method of a branched polycarbonate including a process of, after producing a low molecular weight polycarbonate, adding thereto a polyfunctional compound and mixing (Patent Document 15).
Patent Document 1: JP Patent No. 3102927C
Patent Document 2: JP Hei. 5-202180A
Patent Document 3: JP Hei. 7-18069A
Patent Document 4: JP Hei. 5-271400A
Patent Document 5: JP Hei. 5-295101A
Patent Document 6: U.S. Pat. No. 4,562,242
Patent Document 7: JP Patent No. 324925C
Patent Document 8: JP Patent No. 3997424C
Patent Document 9: JP Patent No. 4598958C
Patent Document 10: JP Patent No. 2674813C
Patent Document 11: JP Sho. 52-36159B
Patent Document 12: JP Hei. 06-099552B
Patent Document 13: WO 2011/062220A
Patent Document 14: WO 2012/108510A
Patent Document 15: WO 2012/005251A